1. Field of the invention.
The present invention relates to the manufacture of a composite solid electrolyte and, more particularly, a composite solid electrolyte comprising a mixture of a solid ionically conductive powder and an ionically conductive polymeric material wherein the powder is maintained in a matrix formed by the polymeric material.
2. Description of the Prior Art.
Solid state electrochemical devices are the subject of intense investigation and development. They are described extensively in the patent literature. See, for example, U.S. Pat. Nos. 4,303,748 to Armand; 4,589,197 to North; 4,547,440 to Hooper et al, and 4,228,226 to Christiansen. These cells are typically constructed of an alkali metal foil anode, an ionically conducting polymeric electrolyte solution containing an ionizable alkali metal salt, and a finely divided transition metal oxide as a cathode.
Solid electrolyte powders which are ionically conductive are disclosed in U.S. Pat. Nos. 4,247,499 to Glugla et al.; 4,388,385 and 4,414,607 to Sekido et al.; 4,394,280 to Von Alpen et al.; 4,432,891 to Susman et al.; 4,539,276 to Harbach; and 4,557,985 to Voss.
The solid electrolyte in Glugla et al. U.S. Pat. No. 4,247,499 comprises a crystalline inorganic material embedded in a polymeric film. The preferred ionic conductive crystal is a beta-alumina crystal. The solid electrolyte in Sekido et al. U.S. Pat. No. 4,388,385 and 4,414,607 is based on a silver halide source. A particularly preferred electrolytic powder has the formula of RbCu.sub.4 I.sub.1.75 Cl.sub.3.25. Matsushita Electric Industrial Company, assignee to the Sekido et al. patents, has developed a paper electrolyte wherein the electrolytic powder is mixed with a high polymer insulator material derived from styrene and butadiene and made into a cement. Von Alpen et al. U.S. Pat. No. 4,394,280 discloses a mixed crystal for use as an ion conducting solid electrolyte which is formed from components of Na.sub.2 O, ZrO.sub.2, P.sub.2 O.sub.5 and SiO.sub.2. Susman et al. U.S. Pat. No. 4,342,891 disclose a glass capable of ionic conduction. The glass is prepared from a non-metal glass former such as GeS.sub.2, B.sub.2 S.sub.3 and SiS.sub.2 in mixture with a glass modifier such as Na.sub.2 S. Voss U.S. Pat. No. 4,557,985 discloses a ceramic solid electrolyte. The electrolyte may take the form of a beta-alumina, a mixed crystal component, or a lithium nitride material.
Although the above-described solid electrolyte powders have been used in electrochemical devices, they suffer as a result of their powdery form. In practice, to produce a solid electrolyte material from the ionically conductive powders, the powders are typically compressed or compacted, by a mold, for example, to form a tightly adherent body. For some materials, particularly glass and ceramic materials, it is nearly impossible to form a unitary structure without the addition of materials such as binders. For other materials where a unitary structure may be produced, the resulting structure is extremely brittle and may be easily fractured as a result of its inflexible physical state.
Prior art references have taught that it is possible to form an electrolyte material by forming an interpenetrating network from a curable material wherein the network functions to house an ionically conductive electrolyte. For example, Bauer et al. U.S. Pat. No. 4,654,279 describes a cell in which the electrolyte is a two phase interpenetrating network of a mechanically supporting phase of a continuous network of a crosslinked polymer and an interpenetrating conducting liquid polymer phase comprising an alkali metal salt of a complexing liquid polymer which provides continuous paths of high conductivity throughout the matrix. In one embodiment, a liquid complex of a lithium salt and polyethylene oxide is supported by an epoxy, a polymethacrylate, or a polyacrylonitrile matrix. The network is formed by preparing a liquid solution of the metal salt, the salt-complexing liquid polymer, and the monomer for the crosslinked supporting phase in a polar solvent. The solvent is evaporated to form a dry layer of a mixture of the remaining materials. The dry layer is then cured.
Le Mehaute et al. U.S. Pat. No. 4,556,614 discloses a solid electrolyte solution for an electrochemical cell in which a salt complexing polymer is mixed with a miscible and crosslinkable second polymer. The function of the second polymer is to maintain the complexing polymer in a more highly conductive amorphous state. This is accomplished by forming a solution of the two polymers and an ionizable salt in a solvent, evaporating the solvent, and crosslinking the second polymer. The second polymer is crosslinked by radiation.
Commonly assigned U.S. patent application Ser. No. 115,492 discloses a polymeric electrolyte wherein a radiation inert ionically conducting liquid having an ionizable alkali metal salt complexed therewith is maintained in a network of a cured photohardened polymer.
Although solid ionically conductive powders are known in the art, and the use of ionically conductive polymeric materials to form a matrix for housing an ionically conductive liquid or solid solution is known in the art, there has been no suggestion that an ionically conductive polymeric material be used to form a matrix for housing a solid ionically conductive powder for improving the mechanical properties of the electrolyte.
Accordingly, there exists a need in the art for a solid electrolyte which has improved mechanical properties without a corresponding reduction in ionic conductivity.